Precise wafer temperature control is vital in all thermally activated steps during an integrated circuit fabrication process sequence. These processes include epitaxial and oxide growth, and dielectric and metal film depositions. The trend towards single wafer processing rather than batch processing requires accurate wafer temperature measurement for successful temperature control. Presently, one of the major impediments for temperature control is the lack of a direct and non-contact measurement of the surface temperature of silicon wafers during processing.
Thermocouples are the most common form of temperature monitoring devices in processing equipment. Thermocouples have several disadvantages. The most significant disadvantage being that they contact the wafer. Poor contact causes a loss of accuracy, and contact with the wafer front side damages the wafer and perturbs the process. Another problem is the lag between the measured temperature and the actual temperature due to the thermal mass of the thermocouple. The thermal mass causes the temperature at the contact point to differ from the true wafer temperature. In addition, there are problems associated with the presence of the metal thermocouple in the harsh environment of the processing chamber. Two such problems are that the thermocouples are attacked by the process gases and are perturbed by RF voltages during plasma processes.
An alternative to thermocouples bonded to the semiconductor wafer is pyrometry. Pyrometry, which offers a direct, non-contact measurement of temperature, runs into difficulties because the emissivity of silicon is a strong function of temperature in the temperature domain of interest. The emissivity is spectrally dependent, and is also dependent on the films on the wafer and surface roughness of the wafer. Thus the emissivity cannot be calculated apriori. This drawback can be overcome with an independent measurement of the emissivity, but this has proven difficult to do. Ellipsometry and variants of ellipsometry, FTIR, the Luxtron-Accubifer RIPPLE technique, and the Quantum Logic technique, are all capable of real-time emissivity measurement, but are not compatible with either multi-zone lamp heating/control or with multi-zone pyrometry, both of which are essential for rapid thermal processing (RTP). Other techniques for emissivity measurement such as monitoring the surface reflectance using an infrared source, are also not compatible with multi-zone pyrometry. Techniques that are based on the thermal expansion of silicon such as the Peak .mu.-Temp approach, or the diffraction monitoring of a probe laser beam by a diffraction grating are not compatible with multi-zone pyrometry, and also need an independent measurement of the initial wafer temperature. Thus there is a need for an approach that combines pyrometry with real-time emissivity measurement and is compatible with multi-zone lamp heating/control and multi-zone pyrometry.